ProgrammingMorphs is an interactive Squeak book that teaches you the basics of programming your own morphs. Install it via SqueakMap, or download and load it. It works under Squeak 3.8, but not 3.9.

It has 6-7 pages in it. Each page describes one aspect of Morphic, gives some code to play with that aspect, and then gives you an exercise to try yourself. It covers the basics of composing morphs and receiving input.